1. Field of the Invention
This invention relates to the field of cementing operations and more specifically to the field of using swelling agents to service a wellbore.
2. Background of the Invention
A natural resource such as oil or gas residing in a subterranean formation can be recovered by drilling a well into the formation. The subterranean formation is usually isolated from other formations using a technique known as well cementing. In particular, a wellbore is typically drilled down to the subterranean formation while circulating a drilling fluid through the wellbore. After the drilling is terminated, a string of pipe, e.g., casing, is run in the wellbore. Primary cementing is then usually performed whereby a cement slurry is pumped down through the string of pipe and into the annulus between the string of pipe and the walls of the wellbore to allow the cement slurry to set into an impermeable cement column and thereby seal the annulus. Secondary cementing operations may also be performed after the primary cementing operation. One example of a secondary cementing operation is squeeze cementing whereby a cement slurry is forced under pressure to areas of lost integrity in the annulus to seal off those areas.
One problem commonly encountered during primary cementing is the presence of one or more permeable zones in the subterranean formation. Such permeable zones result in the loss of at least a portion of the cement slurry to the subterranean formation as the slurry is being pumped down through the casing and up through the annulus. Due to such loss, an insufficient amount of the slurry passes above the permeable zones to fill the annulus from top to bottom. Further, dehydration of the cement slurry may occur, compromising the strength of the cement that forms in the annulus. The permeable zones may be, for example, depleted zones, zones of relatively low pressure, lost circulation zones having naturally occurring fractures, weak zones having fracture gradients exceeded by the hydrostatic pressure of the cement slurry, or combinations thereof. In some cases, the weak zones may contain pre-existing fractures that expand under the hydrostatic pressure of the cement slurry.
Various methods and chemicals have been used in attempts to prevent such problems. For instance, swelling agents have been used to plug such permeable zones by blocking undesirable flow pathways. Such swelling agents typically absorb water and expand to form a mass that plugs the flow pathway. The swelling agents are typically placed downhole at the permeable zone by mixing with a carrier fluid. Drawbacks to such techniques include limitations on the concentration of the swelling agent in the carrier fluid, which typically requires a large quantity of carrier fluid. In addition, pumping large quantities of carrier fluid is typically time consuming. Further drawbacks include premature swelling of the swelling agent, for instance by exposure to water before reaching the intended location in the wellbore.
Consequently, there is a need for more efficient methods of preventing lost circulation. Further needs include a more efficient method of delivering swelling agents downhole. Additional needs include improved methods for plugging permeable zones.